Exercise training promotes cardioprotection against ischemia-reperfusion (I-R) injury. Specifically, endurance exercise training reduces I-R-induced protease (i.e. calpain) activation, oxidative injury, and myocardial infarction. The mechanism responsible for this exercise-mediated cardioprotection remains unclear and is the focus of this proposal. Potential mechanisms to explain the protective effect of exercise include improved collateral circulation, elevation of cardiac heat shock proteins, and increased myocardial antioxidant capacity. Based on published work and preliminary experiments in our laboratory, we postulate that improvements in myocardial antioxidants minimize I-R-induced calpain activation and oxidative injury and are essential for exercise-induced protection against infarction. Specifically, we hypothesize that the exercise-induced protection against an I-R insult is critically dependent on a training-induced increase in both manganese superoxide dismutase (MnSOD) activity and glutathione (GSH) levels in the heart. This hypothesis will be tested in two specific aims. First, we will determine if the exercise-induced protection against I-R-induced calpain activation and oxidative injury is dependent upon an increase in myocardial MnSOD activity and GSH levels. Secondly, we will ascertain if the exercise-induced increase in both myocardial MnSOD activity and myocardial GSH levels are essential for protection against I-R-induced myocardial infarction. Using both pharmacological and molecular approaches (e.g. antisense oligodeoxyribonucleotides to MnSOD), we will independently prevent the exercise-induced synthesis of myocardial MnSOD and GSH. We will then use an in vivo model of myocardial I-R to determine if exercise-induced increases in both myocardial MnSOD and GSH are essential in protecting against the damaging effects of I-R on the heart. These studies will improve our understanding of the mechanisms responsible for exercise-induced protection against I-R damage and will have important implications for management of patients at risk for myocardial I-R injury.